The present invention relates in general to polymer-lithographic processes. Specifically, the present invention is concerned with a process for the fabrication of sub-wavelength structures.
In semiconductor technology and in microelectronics, the dimensions of structures are becoming smaller and smaller. In memory production today, e.g., structures with a width of less than 400 nm are produced using optical lithography in combination with the masking technique. Photholithographic processes are vital steps in the fabrication of, e.g., semiconductor devices. In a photolithographic process, an exposure light, usually ultraviolet (UV) light is used to expose a photoresist-coated semiconductor wafer through a mask (in the following called photomask). The purpose of the photolithographic process is to transfer a set of patterns representative of the circuit layer onto the wafer. The patterns on the photomask define the positions, shapes and sizes of various circuit elements such as diffusion areas, metal contacts and metallization layers, on the wafer.
In optical lithography a limit can be expected at approximately 150 nm because of diffraction effects.
However, structures with even smaller dimensions are required for new applications such as single-electron transistors or molecular electronic components. In the case of very high-frequency circuits this is also true in conventional electronics. There is also a need to reduce, e.g., the read and write dimensions in thin film magnetic heads. In addition to that, micro structures having a very high aspect ratio of about 5 to 30 and greater will be needed.
The resolution of conventional optical lithography schemes is mainly limited by the wavelength of the light used for the transfer of a mask pattern onto a resist. The wavelength of the exposing radiation is a main determinant of pattern resolution W, given by the Rayleigh equation , where l is the wavelength of the exposing light, NA is the numerical aperture of the optical lithography tool, and k1 is a constant for a specific lithography process. In other words, the resolution W is proportional to the wavelength l of the exposing light. Cutting-edge production today creates features that are 130 nm wide, using 248 nm illumination. Currently, the implementing schemes based on light are the bottleneck when trying to obtain structures of a feature size below 100 nm. State-of-the-art optical lithography systems for making current DRAMs, for example, are quite expensive. Alternative processes become attractive when moving on to smaller feature sizes, but the required investments are huge. Thus, techniques that maintain compatibility with existing processes are inherently valuable.
One well-known form of optical lithography is the so-called hard-contact lithography, where a mask is put directly into contact with a substrate to be patterned. Features on the mask, comprising alternatively translucent and opaque regions in a well-defined pattern, are transferred into a photoresist in a 1:1 relation to their dimensions on the mask. Hard-contact lithography can, in principle, make structures with sizes below the wavelength of illumination, but the contact used to place the mask on the substrate compromises the integrity of the process as the possibility of confounding material on the surface of the mask and mask damage greatly limit the useful number of times the mask can be used. Cost is particularly worrisome as the feature scale shrinks and the expense of mask fabrication skyrockets with the increase in the density of its features. Contact masks are also generally much more expensive than those used in optical-projection lithography since for an equivalent resolution the critical dimensions in the former need to be smaller than those in the latter, by the reduction factor used in a projection system. Dust particles and other physical impediments to the substrate are catastrophic in hard-contact lithography as they lift the mask away from the surface, blurring the pattern. Such defects appear over an area much larger than the obscuring particle because the mask is unable to conform around their presence; this problem is compounded as the feature scale shrinks such that even a 200 nm particle can be harmful. In addition, the resist can get stuck to the mask. Hard-contact lithography has thus not found a significant role in manufacturing of small-scale integrated circuits.
There are many approaches known, that improve conventional lithography systems in that filters, projection lenses, or appropriately modified masks are employed. These approaches become increasingly complicated and expensive with reduced feature scale. One example is the so-called optical-projection lithography. The optical lithography based on projection is a very successful and widely employed means of making features down to 200 nm. Here, a pattern of intensity variations in the far field results when light is shone through a mask like that used in contact lithography. The light propagates through air and is focused by a lens to create an image of the desired pattern on a resist-covered substrate, often demagnified by a factor of 5–10 from its size on the mask. Projection lithography is largely limited to features sizes equal or larger than the wavelength I of light. Its implementation becomes increasingly difficult as the scale shrinks towards and below 200 nm, where very complicated systems of lenses and materials are required to carry out existing and proposed schemes. The area over which uniform illumination can be achieved is particularly problematic The maximum current field size with the best 248-nmexposure tool is now only 30×30 mm.
It is generally a disadvantage of most of these approaches that they are getting more and more complicated and expensive when trying to obtain smaller feature sizes. Furthermore, there is a tradeoff between maximum resolution, depth of focus and achievable field image which comes from the use of a lens to focus the light.
European Patent Office publication EP-A-1 001 311 proposes a patterning device with which incident light is guidable at least partially to at least one cover element which is in contact with the patterning device. The cover element comprises light-sensitive material and is arranged on top of a substrate protrusion element on a surface of a substrate and/or is itself structured on a substrate.
Though many approaches have been made to arrive at critical dimensions by using conventional lithography systems, there is still a need for uncomplicated and low-cost methods for small feature generation.
On the other hand, printing from a patterned surface to thin layers of material is a well known and well documented process in printing industry.
Printing processes were originally developed for the exchange and storage of information adapted to human vision. This field of application requires pattern and overlay accuracies down to 20 μm for high-quality reproduction. In a few cases, printing processes have been used for technological patterning, e.g., gravure offset printing was used to make 50-μm-wide conductor lines on ceramic substrates, and to pattern thin-film transistors for low-cost displays. Offset printing was used for the fabrication of capacitors and printed and plated metal lines as narrow as 25 μm. Finally, printed circuit boards and integrated circuit packaging are popular applications of screen printing in the electronics industry. (B. Michel et al., IBM J. Res. Develop. 45, 697 (2001) and references therein).
In a process known as flexography, viscous ink is printed onto porous paper and permeable plastic. Flexography is a direct rotary printing method that uses resilient relief image plates of rubber or other resilient materials including photopolymers to print an image on diverse types of materials that are typically difficult to image with traditional offset or gravure processes, such as cardboard, plastic films and virtually any type of substrate whether absorbent or non-absorbent. As such it has found great applications and market potential in the packaging industry. Usually, the viscous ink prevents a direct contact of the stamp with the substrate because it cannot be displaced quickly enough during the fast printing operations. The transfer of a thick layer of ink is desired in this typical mode of operation but also prevents replication of laterally small features—this is the main reason why printed feature sizes cannot be smaller than 20 μm. Printing onto metal foils has been implemented in a few applications but is much more difficult than other processes (H. Kipphan, “Handbuch der Printmedien”, Springer Berlin, 2000 and J. M. Adams, D. D. Faux, and J. J. Rieber, “Printing Technology 4th Ed.”, Delamare Publishers, Albany, N.Y.).
Microcontact printing uses a similar stamp as flexography does, but typically transfers a monolayer of ink onto an impermeable metal surface. A more general process now called soft lithography is successfully applied in different variants to print thiols and other chemicals to a wide variety of surfaces. Typically, the chemicals are first applied to the patterned stamp surface as solutions in a volatile solvent or using a contact inker pad. After inking and drying, the molecules are present in the bulk and on the surface of the stamp in a “dry” state and are transferred to the surface by a mechanical contact. Reasons for the choice of poly-(dimethyl)siloxane (PDMS) as the stamp material are its good rubber-like elasticity, a chemistry similar to glass, the possibility to buffer ink molecules, and (very important) its excellent gas permeability that enables small amounts of air to dissolve into or escape through the stamp matrix. (see B. Michel et al. “Printing meets lithography”, IBM, J. Res. Develop. 45 (5), 697 (2001)).
There remains a need for a method for the manufacture of sub-wavelength structures that are not diffraction restricted, and particularly for structures having increased aspect ratios, so that existing critical dimensions may be narrowed still further.